Even though the Secret service had to rely on a foreign country security teams for certain information they should have conducted their own "Due Diligence". I even do that when I travel to a foreign country. This obviously was a massive failing on the part of the Secret Service and any other intelligence gathering agency they may rely upon.
"In the hours before President Barack Obama arrived at a Johannesburg soccer stadium to honor Nelson Mandela, the White House staff was in the dark on critical details. here would the president and first lady Michelle Obama sit? When was Obama supposed to speak? Who else would be on stage during the speech?
The result was an array of confusion and security risks that typically would not be tolerated by the Secret Service in the United States. The situation showed just how the Secret Service often is at the mercy of foreign governments to make arrangements when the president is overseas.
There were metal detectors and X-ray machines at the stadium, but they were not used on the initial crowds streaming in for the ceremony, according to Associated Press reporters on scene.
Many people walked through with little or no screening. Inside the stadium there were few signs of the heavy security that routinely would accompany an event with Obama and other world leaders."
Confusion, Security Risks at Nelson Mandela Event - ABC News